东天山康古尔塔格金矿田控矿构造特征

利用构造面理统计、岩组分析和岩石磁化率各向异性参数测量,研究了东天山康古尔金矿田控矿构造的特征。结果表明,金矿田发育一组构造面理,具有典型的对称型岩组图;磁组构特征的变化明显反映出矿田韧性变形具北强南弱的特点;矿田韧性断裂具有压扁应变的特征,磁面理产状与韧性断裂产状相一致。因此,康古尔金矿田韧性断裂应是起源于共轴变形的韧性挤压带。     

Origin and palaeoenvironmental significance of lamination in stalagmites from Katerloch Cave,Austria

The origin and environmental dependencies of lamination in stalagmites from Katerloch, common in speleothems from other cave sites, are examined in detail. Petrographic observations and chemical analyses (including isotopes) of stalagmites and modern calcite were combined with multi‐annual cave monitoring. All investigated stalagmites are composed of low‐Mg calcite and show white, porous laminae and typically thinner, translucent dense laminae. The binary lamination pattern results from changes in the calcite fabric: white, porous laminae are characterized by a high porosity and abundant fluid inclusions and also by enhanced vertical growth and thinning towards the flanks. Translucent, dense laminae exhibit a compact fabric and constant thickness of individual growth layers. U‐Th dating supports an annual origin of the lamination and the seasonally changing intensity of cave ventilation provides a robust explanation for the observed relationships between lamination, stable C isotopic compositions and trace elements (Mg, Sr and Ba). The seasonally variable air exchange, driven by temperature contrasts between the cave interior and outside atmosphere, modulates the rate and amount of CO₂ degassing from the drip water and affects the hydrochemistry and consequently the fabric of the precipitating calcite. Although cave air composition and drip rate are both major variables in controlling CO₂ degassing from the drip water, the seasonally changing ventilation in Katerloch exerts the primary control and the results suggest a secondary (amplifying/attenuating) influence of the drip rate. Drip rate, however, might be the controlling parameter for lamina development at cave sites experiencing only small seasonal cave air exchange. Importantly, the seasonally variable composition of drip water does not reflect the seasonal cycle of processes in the soil zone, but results from exchange with the cave atmosphere. The alternating porous and dense calcite fabric is the expression of a variable degree of lateral coalescence of smaller crystallites forming large columnar crystals. The white, porous laminae represent partial coalescence and form during the warm season: low calcite δ¹³C values are linked to low δ¹³C values of cave air and drip water during that time. This observation corresponds to times of reduced cave ventilation, high pCO₂ of cave air, low drip water pH, lower calcite supersaturation and typically high drip rates. In contrast, the translucent, dense laminae represent more or less complete lateral coalescence (inclusion‐free) during the cold season (high calcite, drip water and cave air δ¹³C values), i.e. times of enhanced cave ventilation, low cave air pCO₂, increased drip water pH, relatively high calcite supersaturation and typically low drip rates. In essence, the relative development of the two lamina types reflects changes in the seasonality of external air temperature and precipitation, with a strong control of the winter air temperature on the intensity of cave‐air exchange. Thick translucent, dense laminae are favoured by long, cold and wet winters and such conditions may be related closely to the North Atlantic Oscillation mode (weak westerlies) and enhanced Mediterranean cyclone activity during the cold season. Studies of speleothem lamination can thus help to better understand (and quantify) the role of seasonality changes, for example, during rapid climate events.     

钦杭成矿带南段盘龙铅锌矿成矿体系环境分析—来自矿石组构的限定

盘龙铅锌矿位于钦杭成矿带南段,为近年来新发现的大型喷流-沉积(SEDEX)型铅锌矿床。矿体严格受控于泥盆纪上伦组白云岩。本文基于岩相学研究,系统分析了盘龙铅锌矿矿石组构特征,识别出SEDEX原生成矿阶段形成的条带状、条纹状、浸染状、草莓状、(变)胶状沉积构造,后期改造的网脉状、角砾状、环带构造以及交代、压力结构。本研究认为,成矿包括原生沉积期、活化改造期和地表氧化期,其中原生沉积期又可分为白云石-黄铁矿阶段和硫化物沉淀阶段。扫描电镜(SEM)分析显示,盘龙铅锌矿多数硫化物颗粒很细小,黄铁矿存在草莓状、胶状、五角十二面体的不同形貌特征,重晶石存在浑圆细粒状、放射状、脉状三种形貌,表征盘龙铅锌矿的成矿过程中体系环境经历了温度先升后降,p H先降后升,氧化性逐渐降低的变化。     

Assessment of rock geomechanical quality by quantitative rock fabric coefficients: Limitations and possible source of misinterpretations

Rock microfabrics were quantified by image measurement system (petrographic image analysis) of thin sections. Along with common measured parameters (grain area and size, length of major and minor axes and their orientation), some quantitative parameters (aspect ratio, grain boundary smoothness) and fabric coefficients (micropetrographic quality index (K), “texture” coefficient (TC), indexes of interlocking (t) and grain size homogeneity (g)) were determined. The reliability of these parameters for the evaluation of geomechanical properties was tested on the set of granitic rocks showing pronounced variability in rock mechanical properties. None of the tested rock fabric coefficients or parameters proved a close correlation to rock mechanical properties except grain size. Based on experimental findings, the grain size is, along with porosity, the controlling factor of rock mechanical properties of genetically and mineralogically similar rocks. Obliteration of the real effects of rock fabric parameters on the mechanical properties of rocks by application of complex fabric coefficients disables their practical use in petrography applied to geomechanics.     

Modelling the behaviour of an embankment on soft clay with different constitutive models

The paper investigates the effect of constitutive models on the predicted response of a simplified benchmark problem, an embankment on soft soil. The soft soil is assumed to have the properties of POKO clay from Finland and five different constitutive models are used to model the deposit. Two of the models are isotropic models, i.e. the Modified Cam Clay model and the Soft‐Soil model. The other models are recently proposed constitutive models that account for plastic anisotropy. The S‐CLAY1 and S‐CLAY1S models are embedded in a standard elasto‐plastic framework and account for anisotropy via a rotational hardening law. In addition, the S‐CLAY1S model accounts for bonding and destructuration. In contrast, the Multilaminate Model for Clay (MMC) accounts for plastic anisotropy by utilizing so‐called multilaminate framework. The results of numerical simulations show that accounting for anisotropy results in notable differences in the predicted settlements and horizontal movements compared to the predictions using the isotropic models. There are also significant differences in the K₀ predictions by the different constitutive models and this has a significant impact on the results. Copyright © 2006 John Wiley & Sons, Ltd.     

A micromechanical study of rolling and sliding contacts in assemblies of oval granules

The evolution of the microstructure of an assembly of cohesionless granular materials with associated pores, which carry the overall applied stresses through frictional contacts is a complex phenomenon. The macroscopic flow of such materials take place by the virtue of the relative rolling and sliding of the grains on the micro‐scale. A new discrete element method for biaxial compression simulations of random assemblies of oval particles with mixed sizes is introduced. During the course of deformation, the new positions of the grains are determined by employing the static equilibrium equations. A key aspect of the method is that, it is formulated for ellipse cross‐sectional particles, hence desirable inherent anisotropies are possible. A robust algorithm for the determination of the contact points between neighbouring grains is given. Employing the present methodology, many aspects of the behaviour of two‐dimensional assemblies of oval cross‐sectional rods have been successfully addressed. The effects of initial void ratio, interparticle friction angle, aspect ratio, and bedding angle on the rolling and sliding contacts are examined. The distribution of normals to the rolling and sliding contacts have different patterns and are concentrated along directions, which are approximately perpendicular to one another. On the other hand, the distribution of all contact normals (combined rolling and sliding) are close to that of rolling contacts, which confirm that rolling is the dominant mechanism. This phenomenon becomes more pronounced for higher intergranular friction angle. Characteristics of the rolling and sliding contacts are also discussed in the context of the force angle, which is the inclination of contact force with respect to the contact normal. Copyright © 2003 John Wiley & Sons, Ltd.     

K0 estimation in level granular soil from anisotropic wave velocities on the basis of micromechanics

This study aims to explore the possibility for estimating K₀ in a level ground of granular soil by seismic methods on the basis of micromechanics theory. The idea was to simulate in situ cross‐hole seismic method for the measurement of wave velocities along various directions of wave‐propagation. This work made use of a field simulator to control a K₀ condition (zero lateral strain condition) in specimens. A series of vertical loading containing subsequent loading and unloading were applied to the specimen prepared by pluviation. In general, the K₀ values determined experimentally in this work agreed with the exiting empirical relations. K₀ value was also calibrated from measured anisotropic wave velocities using an optimization procedure. From the comparison of the back‐calculated and measured results of K₀, it revealed the feasibility for the determination of the in situ lateral stress in granular soil by seismic methods and on the basis of micromechanics theory as long as enough wave‐velocity measurements along various directions of wave‐propagation were available. The potential for the usage of the presented methodology for the determination of the in situ lateral stress in level‐ground of granular soil by seismic methods seems encouraging. Copyright © 2004 John Wiley & Sons, Ltd.     

A non‐coaxial constitutive model for sand deformation under rotation of principal stress axes

A constitutive model for the simulation of non‐coaxiality, an aspect of anisotropic behavior of sand subjected to the rotation of the principal stress axes, is presented in this paper. Experimental studies have shown that non‐coaxiality or non‐coincidence of principal plastic strain increments with principal stress axes under loadings involving the rotation of principal stress axes may be considerable. Besides, the rotation of the principal stress axes results in dramatic effects on stiffness and dilatant behavior of sand. Therefore, the consequences of principal stress axes rotation on deformational behavior, dilatancy and soil stiffness must be taken into account in theoretical and practical problems. To this aim, the following steps are taken: (1) A general relationship for flow direction with respect to possibility of non‐coaxial flow is developed. Moreover, special circumstances linking non‐coaxiality to instantaneous interaction between loading and soil fabric are proposed. (2) Proposing novel expressions for plastic modulus and dilatancy function, the model is enforced to provide realistic simulations when sand is subjected to the rotation of the principal stress axes. Finally, with numerous examples and comparisons, the model capabilities are shown under various stress paths and drainage conditions. Copyright © 2007 John Wiley & Sons, Ltd.     

循环荷载下砂土液化特性颗粒流数值模拟

利用PFC2D常体积循环双轴试验条件,对砂土在不排水循环荷载作用下的液化特性进行了颗粒流数值模拟,数值模拟按等应力幅加荷方式进行。颗粒流数值模拟的优点在于得到试样液化宏观力学表现的同时,通过不同循环加荷时刻试样内细观组构参量（包括配位数、接触法向分布、粒间法向接触力、粒间切向接触力）的演化规律,分析砂土液化过程中细观组构变化与宏观力学响应之间的内在联系,从而可进一步探讨砂土液化的细观力学机制。数值模拟研究结果表明,砂土液化现象在宏观力学表现上反映为超静孔隙水压力的累积上升和平均有效主应力的不断减小,在细观组构上对应于配位数的累积损失和粒间接触力的不断减小。砂土液化细观机制分析表明,试样配位数的减少与循环加荷过程中组构各向异性滞后于应力各向异性有关。